Diffusion-cooled, gas-discharge slab lasers are characterized by output power of several watts per unit area of discharge. Slab gas lasers based on large area electrodes confining a CO2 laser mixture, driven by radio-frequency (RF) power supplies (FIG. 1A), are currently delivering over three kilowatts of power with excellent beam quality, due to a special optical resonator, allowing for maximum exploitation of the discharge gain volume for a single mode beam amplification.
In order to maximize the discharge cooling, which is of prime importance for electrically excited gas lasers, the planar discharge width is of the order of the planar electrodes gap, which is typically 1-2 mm. In addition, the discharge electrodes are used as an optical waveguide for the amplified laser beam. Stabilization of the discharge is achieved by feeding the discharge with over 100 MHz RF power, with a wavelength comparable to the electrodes longitudinal dimensions and thereby homogeneous discharge is achieved.
The laser output power is limited by the allowed RF power load in the discharge per unit area. This power load is inversely proportional to the electrodes spacing. Exceeding the power load limitation for a given electrodes spacing, results in laser deterioration, due to thermal deleterious effects.
However, further reduction of the electrode spacing for allowing higher RF power load in the discharge is prohibited due to the following reasons:
1) At 100-200 MHz excitation frequency, it is well known to those skilled in the art that the discharge plasma is divided into two sections in a stable, typical RF discharge (FIG. 1B): depletion layers located in the vicinity of the electrodes, in which positive ions are present and electrons are drifted to the electrodes, prohibiting the possibility for laser gain, and a positive column, layer or zone in which neutral plasma is achieved and a laser gain is created, due to efficient electron excitation of the active gas.
The depletion layers width is inversely dependent on the RF excitation frequency. At 125 MHz excitation frequency, the depletion layers width is of the order of 400 μm. Reducing the electrode gap toward this value will practically reduce the width of the gain holding plasma column and the laser efficiency will be reduced.
2) The electrodes are used as an optical waveguide for the amplified laser beam (FIG. 1C). The electrodes are usually metallic with a dielectric layer implemented on its surface in order to avoid contamination of the plasma and corrosion of the electrodes. It is well known to those who are skilled in the art that the hollow waveguide losses, due to leaky guided modes, are proportional to 1/d3, where d is the electrode spacing (Degnan J. J. The Waveguide Laser: A Review, Appl. Phys., 1976, v. 11, p. 1-33.). Therefore, reducing the electrodes spacing will result in an extensive loss and laser efficiency deterioration.
One of the well-known ways for allowing electrode space reduction and alleviating the first limitation is by increasing the RF frequency. Attempts have been made to excite gas lasers with microwave (MW) discharges, but the waveguide-loss-limited electrode spacing, on the order of 1-2 mm, did not allow for stable CW microwave discharges (due to the narrow depletion layers) and pulsed operation was utilized for achieving stable plasma. In order to allow for stable CW microwave (MW) discharges, a reduction of the electrode spacing is required.
Therefore, reduction of the electrodes spacing for allowing for higher RF power load in the discharge and increasing the laser power per unit electrodes area, necessitates the increase of the excitation RF frequency towards the MW frequency range. In addition, a way for alleviating the laser waveguide losses, due to the reduction of the electrode spacing, is required.
As mentioned before, reducing the electrode spacing in the traditional leaky waveguide, as shown in FIGS. 1A-1C, will result in extensive waveguide losses. As an example, a simulation of a leaky waveguide made of ZnSe (zinc selenide) (refractive index n=2.4 @λ=10.6 μm) confining a CO2 plasma gain volume with a width of 110 μm, is shown in FIGS. 2A-2C. The leaky waveguide losses amount to over 90% loss of power per 1 m of 10.6 μm light propagation through the waveguide.